I know this is not strictly retro but... on my downhill bike I'm having problems with the gear system, all the gears shift back and forth 1st to 9th no problem but in 4th and 5th they slip when I start to pedal on just those 2 gears and I've no idea why. I've put new outer cable on as the old was a bit battered and changed the inner as well as that was wrecked but still keeps slipping in those 2 gears. The tension on the gable is fine the high and low screws are set spot on and I've looked at the cassette and the teeth on the 4th and 5th sprocket don't look any more worn the rest. Any ideas its starting to really get on my nerves.

Yeah cassette is sram og something or the other (the 40quid one so not a cheapy), sram 9.0sl rear mech and shifter, only thing new is the chain which is a kmc x9 which supposed to be really good but it only does it on those 2 gears.

I'm surprised it's not jumping on the smaller sprockets on the smaller half on the block. I guess you've used the 4th & 5th cogs the most and they're the most worn, that's why it only slips on them.

2 ways forward - 1, stick with it and avoid 4 & 5 until the chain has bedded in a little and stops slipping. 2 Buy a new cassette end enjoy perfect shifting again.

I have always replaced chains and blocks at the same time for this very reason. We always tried to get customers in the shop where I used to work to repace them together too. Just avoids problems like you're describing.

Outers are fine Agency plenty enough length on them I always allow for a bit more length on that with it being full suspension and having quite a bit of it. Don't know how many miles the cassette has seen as I brought that second hand on ebay but doesn't look that bad, so the chain should eventually bed in then and be fine?. Just the teeth on those 2 sprockets dont look any more worn than the others.

If you've only just fitted the new chain then I doubt you would have done anything to damage it that quickly. I think changing the cassette is your only option. If that doesn't fix it then I have no idea what the problem is!

I have a sneaky suspicion that the cassette you bought may have come from a bike that has been used predominantly on a turbo trainer? I find the best gear on mine is 'big ring to middle of the block' and then set the turbo trainer to an easier resistance (to account for the big gear). This way the turbo trainer rolls smoother. I also tend to only use a couple of gear on the block around the "sweet spot" in terms of the perfect resistance.

Using the middle of the block also gives a nice straight chain line. I'm probably wrong, but you never know.... Maybe it was used as a single speed on those two sprockets ?

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